1892.] which have been exposed to High Temperatures. 397 



rather brilliant tints with the crossed nicols. The crystals are 

 occasionally composite, some are fairly idiomorphic, others appear to 

 have suffered more or less external corrosion. There are besides 

 some grains of quartz (also affected by the action of the magma) 

 which contain fluid cavities with a rather stream-like arrangement. 

 They are similar to, but generally larger than, those in the ground- 

 mass. Both minerals occasionally enclose microliths, among them, I 

 think, zircon. Besides the mica, we find one or two rather tufted 

 groups of a prismatic mineral, which exhibits a fairly marked 

 dichroism, pale-brown to bluish-green, at first sight not unlike 

 tourmaline. As, however, the extinction certainly is oblique, though 

 the angle is not large, this mineral is no doubt a hornblende, and 

 probably of secondary origin. 



The account may be completed by quoting the following bulk 

 analysis of the rock, given by Mr. Ward (* Geol. Soc. Quart. Journ.,' 

 vol. 32, p. 22) : 



100-000 



The vitrified specimens are coated externally with a pellicle of 

 dark-brown glass ; the surface of one being rather scoriaceous, and 

 " pitted " with cavities full one- third of an inch in diameter ; that is, 

 the ordinary surface of broken felstone (for I presume this to be the 

 outside of a fragment) is replaced by that of a vesicular slag. 

 Internally many minute cavities, from the size of a small pin-head 

 downwards, have been developed; the rock has assumed a darker 

 gray colour, in which the white porphyritic crystals of felspar stand 

 out more distinctly than before;* and it has a more scoriaceous 

 aspect. 



On examining the slides cut from the partially melted rock, two 



* In the specimens of unaltered rock the larger crystals of felspar are of a 

 yellowish tint ; the smaller, however, are white, but less distinct, owing to the pale 

 gray matrix. 



VOL. L. 2 B 



